Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves.

Academic Article


  • Previous spacecraft studies showed that stormtime poloidal ultralow-frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0-5.5 and had peak power at 4-10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters.
  • Authors

  • Takahashi, Kazue
  • Crabtree, Chris
  • Ukhorskiy, AY
  • Boyd, Alexander
  • Denton, Richard E
  • Turner, Drew
  • Gkioulidou, Matina
  • Vellante, Massimo
  • Spence, Harlan
  • Status

    Publication Date

  • February 2022
  • Published In


  • Van Allen Probes
  • compressional ULF waves
  • geomagnetic storms
  • gradient driven instability
  • inner magnetosphere
  • symmetric mode structure
  • Digital Object Identifier (doi)

    Start Page

  • e2021JA030115
  • Volume

  • 127
  • Issue

  • 2